Transmitters that include linear transmitter amplifiers are adapted to operate at a given optimal load with which they can deliver maximum output power and still remain linear. They have respective maximum voltage values and maximum current values, Umax and Imax that they should not exceed. If the impedance of the load on a final stage of an amplifier becomes too low and an attempt is made to adjust to a nominal output voltage, the stage may become non-linear and over-modulated. The same applies when attempting to adjust to a nominal output current when the load impedance is too high.
Such linear transmitter amplifiers, particularly in respect of transmitters intended for portable or mobile use, are often subjected to excessively large and varying standing wave ratios (SWR) in respect of the load and therewith become non-linear. A typical example in this case is when the load is in the form of an antenna which when used in connection with a mobile transmitter (e.g. a mobile telephone) constantly generates different SWRs, depending on how the telephone is held, on the extent to which the antenna is covered by the hand holding the telephone the antenna, and so on. Ideally, the impedance is constantly nominal and no reflected waves occur. Since the load cannot, of course, be kept ideal, for aforesaid reasons, the load impedance of the transmitter will become higher or lower than the ideal load impedance. In case of excessively large variations, the amplifier will lie outside its linear range. Consequently, an amplifier in such a transmitter will be readily steered out beyond its tolerance levels. This results in increased out-channel radiation from the transmitter and causes the modulation to fall outside the tolerances.
Those attempts hitherto made to correct these problems have involved a number of solutions in which the final stage is isolated from the variable load with the aid of isolators or circulators. In simple terms, an isolator or circulator functions by not allowing any reflected power to return to the amplifier from the load, but is terminated instead. Unfortunately, a circulator or isolator is expensive, large and results in power losses.
U.S. Pat. No. 4,547,746 A teaches another known technique that uses a so-called directional coupler where an amplifier is adapted to change its character, whereafter the impedance is changed so as to match the instant value of the load at the amplifier output. This is implemented by virtue of so arranging a feedback signal from the amplifier output so as to obtain a signal that represents the difference between forwarded and reflected power. This signal is compared with a signal which shows the power required to obtain a voltage control signal that adjusts the supply voltage to the transistors in the amplifier. This prevents over-modulation or over-excitation and therewith provides load protection. The concept is also aimed at improving the efficiency of the amplifier.
Another well known technique uses an SWR detection circuit for monitoring the load impedance concerned, wherein a control signal initiates a decrease in the power output from the amplifier to a level that lies inwardly of the set threshold values. This technique, however, is seen primarily as a safety measure for preventing overloading and subsequent damage, rather than a measure for retaining the linearity of the amplifier.
There is therefore desired an amplifier which is able to retain its linearity in a transmitter, even when the load impedance varies significantly, and therewith prevent the over-modulation that can result therefrom.